Case Report: Front surface eccentricity of scleral lens: Its impact on visual quality in eyes with higher-order aberrations induced by refractive surgery

Introduction

Small incision lenticule extraction (SMILE) is a keraratorefractive procedure that is popular as an effective surgical management of common refractive errors.¹ The central corneal curvature is modified, making it flatter to correct myopia and steeper to correct hyperopia. Thus, influencing the corneal optical quality and producing aberrations leading to distorted images. SMILE may increase the total higher-order aberrations (HOAs) which are thought to be responsible for poor quality of vision, even with visual acuity (VA) of 20/25 or better.² Many visual symptoms like starbursts, shadowing, reduction in contrast sensitivity and ghost images postoperatively may be secondary to an increase in the HOAs.³ Total HOAs induced after Wavefront-guided (WFG) keratorefractive procedures are similar to SMILE.² Scleral contact lenses (SCLs) are large-diameter lenses that vault over the entire cornea, mask the corneal irregularity, and enhance vision in irregular corneas.⁴ They are known to effectively neutralize HOAs induced by the anterior corneal surface.⁵ The use of different eccentricities on the front surface of SCLs has been shown to improve the visual performance in eyes with irregular cornea by reducing the HOAs.⁶ This case demonstrates the impact of different FSEs in the Boston Sight Scleral (BSS) lens (Boston, Massachusetts, USA) on ocular aberrations and visual acuity, and also hypothesizes how eccentricity of SCLs can be calculated according to the asphericity of the posterior corneal surface.

Case report

A female in her 20s presented with complaints of diminution of vision for both near and distance for the last 4 months associated with shadowing and ghosting of images after keratorefractive surgery. She had undergone SMILE surgery for myopic refractive error of -4.50 Dioptre Sphere (DS) in the right eye (RE), and -8.00 DS in the left eye (LE) 6 months back.

Her uncorrected high contrast distance visual acuity (HCDVA) recorded under room illumination using the COMPlog chart (COMPlog Clinical Vision Measurement Systems Ltd, London, UK) was 20/60 and 20/40 in RE and LE respectively. The near VA using the English Chart under room illumination was noted to be N6 @ 25 cms in both eyes (BE). After subjective refraction, her HCDVA improved to 20/20p in RE and 20/25 in LE. With pinhole, VA in LE improved to 20/20. Even with the best-corrected vision, her complaints of shadowing of optotypes, and ghosting of images persisted.

On slit-lamp biomicroscopy, the cornea in BE revealed a well-demarcated ring-shaped area of ablation with faint scarring at the anterior stroma. Intra-ocular pressure and posterior segment were within normal limits in BE.

Corneal topography (Allegro Oculyzer, WaveLight, GmbH, Erlangen, Germany) (Figure 1A and B) showed average flat and steep keratometric readings. The front and back surface asphericity (@6 mm) for RE was 0.47 and 0, and for LE was 1.57 and -0.03. iTrace (Tracey Technologies Corp. TX, USA) performed under scotopic conditions keeping the fixed scan of diameter 3.2 mm showing HOAs of 0.230 μ in RE and 0.237 μ in LE. Anterior segment optical coherence tomography (AS-OCT) (Optovue, Inc., Fremont, CA) showed epithelial irregularity bilaterally. With complaints of ghosting of images, and unwillingness to use spectacles, the patient was advised for a contact lens trial.

Figure 1. Corneal topography of the right (1A) and the left eye (1B) showing the central area of flattening post SMILE surgery.

It shows the posterior surface asphericity (Q-value) for the right (0.00) and the left eye (-0.03).

Figures 2 and 3. Digital photograph of the same patient with the final scleral lens fit in the right (Figure 2) and in the left eye (Figure 3) in primary gaze (2A, 3A), downgaze (2B,3B), upgaze (2C, 3C), and lateral gazes (2D, 3D and 2E, 3E) showing optimal scleral lens haptic and edge alignment.

Both conventional RCL (Purecon, New Delhi, India) and Rose K2 lenses (Menicon Co. Ltd, Nagoya, Japan) were tried with no significant improvement in visual quality. Consequently, the patient was advised BSS lens trial. The initial trial lens was selected based on the company’s guidelines to achieve optimal fitting characteristics which included a central vault ranging from 200-400 microns, and a limbal vault ranging from 50-100 microns with well-aligned lens haptics resting on the sclera and overlying conjunctiva. The specifications of the selected BSS lenses were 18.5 mm diameter, 2.80 mm sagittal height, 8.00 mm base curve, with standard haptics. All trial devices shared similar parameters except their eccentricity, which varied from 0, 0.6 and 0.8. Initially, a lens with 0.6 eccentricity (FSE 1) was tried followed by 0 (FSE 0) and lastly, 0.8 (FSE 2), while keeping all other parameters constant. The patient was given an adaptation time of 15 minutes after the application of each lens, after which visual acuity was noted followed by over-refraction. Figures 2 and 3 illustrate the final SCL fit prescribed for the patient. Ocular aberrometry was performed using iTrace with SCLs of different eccentricities. Between two consecutive lenses, a gap of 10 minutes was given. All these measurements were recorded with a lens in situ under scotopic conditions with a scan size diameter of 3.2 mm. The best corrected HCDVA of 20/20 and a subjective reduction in image ghosting were observed with an FSE 0 lens. Table 1 summarizes the impact of various FSEs on HOAs and HCDVA. Since both the quality of vision and HOAs improved with the FSE 0 lens, these lenses were ordered.

Discussion

While keratorefractive surgeries are designed to reduce pre-existing aberrations, they can also induce newer aberrations affecting visual quality and contribute to patient dissatisfaction.⁷ Although WFG enhancement surgeries may mitigate these aberrations, many patients are either reluctant to undergo a repeat surgery or deemed unfit for it due to the associated risk of post-surgical ectasia.

RCLs have been widely used for visual rehabilitation in patients with irregular corneas. The tear film between the lens and the cornea effectively masks corneal irregularity. They not only correct refractive errors but also eliminate third to fifth-order and total HOAs.⁸ Fitting corneal lenses can be particularly challenging in oblate corneas. Reverse geometry lenses can give a better fit, however, the continuous gliding movement of these lenses on the cornea with blinking can lead to instability and discomfort. In our case, the RCLs were neither tolerated nor improved visual quality.

SCLs provide better comfort and tolerability than RCLs. Kumar et al have shown that SCLs can effectively reduce HOAs by 70-80% in eyes with penetrating keratoplasty, radial keratotomy, and in eyes with post-LASIK ectasia.⁹ Hastings et al have shown that WFG SCLs provide superior visual and optical rehabilitation than conventional SCLs.¹⁰ Use of these customised SCLs will need technical expertise, and a sub-micro precision customised lathe-cutting machine, which can be challenging to manufacture, and may not be accessible to all clinicians.

Jagadeesh et al reported that an FSE of 0.60 maximized visual performance and reduced HOAs in moderate keratoconus.¹¹ Hussion et al also observed enhanced high and low-contrast visual acuity with different eccentricities.¹² Similarly, Badrinarayanan et al noted improvements in HOAs, corrected distance VA and contrast sensitivity in keratoconus patients with varying eccentricities.¹³ Despite these findings, an optimal FSE value has not been established.

HOAs due to the anterior surface will get neutralized with SCL but those due to the posterior surface remain.¹⁴ In that case, FSEs may compensate for aberrations due to the posterior corneal surface. FSE also helps in aligning the optical axes of the eye and the optic portion of the SCL which may contribute to visual improvement.¹²

The thickness of the fluid reservoir,¹⁵ the lens material,¹⁶ and the polymer coating¹⁷ have minimal impact on HOAs. Considering the irregularities of the corneal anterior surface, and the anterior and posterior asphericity of the cornea, we hypothesize that a lens with FSE closely matching the eccentricity (Ɛ) of the posterior surface of the cornea should be selected as the first lens for trial. In our case, the posterior asphericity (Q) was noted to be 0 and -0.03 in RE and LE, respectively. The formula Q = –Ɛ² can be used to convert asphericity into eccentricity,¹⁸ resulting the values of 0.00 for RE and 0.17 for LE. Consequently, the selected SCL for the trial was chosen based on the closest FSE, which was 0 for both eyes.

In this case, HOAs were altered by all the lenses. However, it is crucial to consider the patient’s ability to distinguish the target. Our patient perceived improved vision with FSE 0, even though HOAs were further reduced with FSE 0.6. This observation can be demonstrated by the simulation of the ‘E’ optotype depicting the point spread function.

Further studies on corneas with varying posterior surface eccentricities and larger sample sizes are necessary to validate our hypothesis. Until any strong correlation is established, it is advisable to try lenses with different eccentricities before finalizing a lens. The inability to achieve pin-hole VA and complaints of ocular distortion despite optimal refraction can be indicators of the need to incorporate different eccentricities in the SCL. Furthermore, quantifying ocular aberration is crucial. This enhances our understanding of the patient’s experience of blurred vision and also facilitates comparison of the effects of different FSEs.

Conclusion

This case illustrates the influence of different FSEs of BSS devices on visual outcomes following SMILE surgery. SCL with varying FSEs helps to further mitigate these aberrations, thereby enhancing optical quality. The insights gained from this case may facilitate the selection of the first trial lens, considerably reduce the chair time and guide in prescribing lenses with appropriate FSE in post-keratorefractive surgery.

Consent

Written informed consent for publication of their clinical details was obtained from the patient.